By Grant Laidlaw

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding and skills necessary to work in our industry are simply not in place.

Mr Grant, how do we set a fridge that only works on a low-pressure switch; how do we set the switch and temperature if there is no thermostat? Thank you.

Khulani asks:, Your Content Goes Here

Hi Khulani, this situation is found in older walk-in fridges and underbar fridges, although I have seen some new underbar fridges working on this principle. These systems are out in the field and do not have electronic temperature controllers. In these instances, a low-pressure (LP) pressure switch does not only operate as a safety device, but also as a control device.

This means that in these systems, the LP switch in reality, controls the temperature of the installation and is, as a consequence, critical to the commissioning of the unit.

Khulani, in the last issue I described the procedure for setting the LP switch in practice. However, the question remains, how does one determine the settings needed to obtain the desired installation temperature?

Given that what we want to achieve is to have the system run and cycle correctly to achieve a specific temperature, we need to determine the cut-in and cut-out pressure settings for the setting of the pressure switch.

The following represents a tried-and-tested method of calculation and the setting of the LP switch in practice. When compared to what I call the ‘thumb suck method’, the time saving is considerable. With all guesswork removed, the accuracy and improved quality of workmanship is well worth the effort. In addition, the understanding of the system that this method affords the technician is invaluable.

This type of system is used for refrigeration as opposed to freezing and is typically used for the storage of cold meats, cheeses, beverages or anything that does not require freezing. This type of system does not pump down, nor have defrost elements and associated controls, but runs on what is called the automatic defrost cycle.

I have used R134A as an example but using the correct pressure temperature chart enables this method to be used for any refrigerant.

On a system having an installation temperature of 5°C, the automatic defrost cycle operates as follows:

  • During the ‘on’ cycle, the coil is usually at a temperature below the freezing point of water (0°C). This will cause a frost accumulation on the evaporator.
  • During the ‘off’ cycle the evaporator warms up enough (the evaporator fan remains running with the air temperature blowing through the coil at 5°C) so that the frost melts from the coil before the compressor starts again.

To enable the frost to melt during the ‘off’ cycle, it is obvious that the installation temperature must be above the melting point of ice (in my example 5°C). As a general rule, the minimum installation temperature of a system operating on the automatic defrosting cycle is 1°C.

Installation temperature control by means of a LP switch: The ‘pressure-temperature’ relationship of a refrigerant and the following formulae are applied in determining the LP switch settings in order to control the average temperature of an installation.


Box temperature / installation temperature (BT)
Evaporator temperature (ET)
Temperature difference (TD) always given in ‘K’.
BT = ET + TD
TD = BT – ET
ET = BT – TD

Assume that we have a cold room controlled by a pressure switch and fitted with a forced draft evaporator connected to a condensing unit. The desired cold room temperature (BT) is 3°C, and automatic defrosting is required. The plant has been designed to give a temperature difference (TD) of 9K, that is the difference in temperature between the air in the cold room and the refrigerant boiling off in the evaporator (ET) is 9K. The system runs on R134a.

(ET) = (BT) room temperature (3°C) – TD (9K) = – 6°C Refrigerant temperature in evaporator (ET).
Referring to the R134a pressure temperature chart the pressure equivalent to a temperature of – 6°C is 135kPa.
The suction pressure, however, does not remain constant. The average refrigerant pressure is the pressure of the refrigerant in the evaporator taken halfway through the running cycle as measured on a time basis. This should not be confused with the halfway point between the cut-in and cut-out pressure. Referring to the temperature/pressure chart, the average refrigerant temperature is obtained. By subtracting the average refrigerant temperature from the installation temperature, the operating TD of the installation is obtained. The point at which the pressure switch must cut out, is below that of the average suction pressure.
A general rule which may be followed is that the average suction pressure is usually about 14 kPa above the cutout point, excluding the pressure drop in the coil and suction line. This point is called the minimum suction pressure which in this case is then 135 kPa – 14 kPa = 121 kPa.
This is the refrigerant pressure at the cooling unit whereas the pressure switch is located at the condensing unit. Due to the effect of gas friction losses in the coils there is a pressure drop in the evaporator and suction line. This pressure drop in an average installation is about 21 kPa, made up of approximately 14 kPa through the suction line and an additional 7 kPa through to the mid-point of the evaporator. To calculate the suction pressure at the condensing unit, we must subtract the pressure drop in the coil and suction line (21kPa).
The minimum suction pressure at the condensing unit for a 3°C installation temperature = 121 kPa – 21 kPa = 100 kPa. This figure is then the cut-out point of the pressure (100kPa) The cut-in point of the pressure switch is determined by the room temperature when automatic defrosting is required.
Water freezes and ice melts at approximately 0°C, but in practice a slightly higher temperature is necessary to ensure fairly rapid defrosting of the evaporator. As the required room temperature is 3°C, the evaporator can safely be allowed to reach about 4°C during the defrost cycle.
Again, referring to the pressure temperature chart, we find that the equivalent pressure of R134A for 4°C = 229 kPa. If the cut-in point of the pressure switch is set at approximately 229 kPa, automatic defrosting will result.
Assuming that we have a cold room fitted with a forced air evaporator connected to a condensing unit, the desired cold room temperature (BT) is 3°C and automatic defrosting is required. The plant has been designed to give a temperature difference (TD) of 9K. This gives an ET of – 6°C converted to pressure equals 135 kPa.

Cut-out pressure = ET – General rule – Suction loss
= 135 kPa – 14 kPa – 21 kPa
= 100 kPa cut-out pressure

Cut-in pressure = BT + 1°C (defrost cycle)
= 3°C + 1°C
= 4°C (converted to kPa)
= 229 kPa cut-in pressure

Setting the pressure switch to these values will achieve the desired installation of a correctly designed system.

If the evaporator TD is unknown, you can determine the value as follows:

The first step in checking the TD of any installation is to determine the fixture or box temperature (BT). This is very easily done by using a thermometer to check the temperature of some article which has been in the fixture for at least 10 hours. In this way a peak temperature reading (either high or low) will be avoided.

The next step is to determine the average refrigerant temperature (ET). For example, consider a kitchen reaching refrigerator using R134A on which the switch is set to cut-out at 90 kPa and to cut in at 234 kPa to hold a cabinet temperature of 3°C.

As discussed before, a general method which may be followed in determining the average refrigerant temperature is that the average suction pressure is usually about 35 kPa, (14 kPa general rule + 21 kPa suction loss), above the point at which the switch cuts out. The temperature corresponding to this pressure may then be assumed to be the average refrigerant temperature. Therefore, the average refrigerant pressure for the above application is:

90 kPa + 35 kPa = 125 kPa. Therefore, the average refrigerant temperature (ET) will be 7°C.

Now to determine the TD on this application we merely subtract the 7°C from the 3°C fixture temperature and we find the TD to be:

TD  = BT – ET
= 3°C – (7°C)
= 10K

Another method typically used on a new installation where no product is present or when for some reason the average box temperature cannot be determined is: After allowing the installation to run for approximately 10 to 15 minutes, measure the temperature of the air entering the coil (on coil temperature). Measure the temperature of the air leaving the coil (off coil temperature). The difference between the two is the TD.

The technician should be familiar with these TD figures, as he will check many installations to determine whether or not a job is properly adjusted for the best food-preserving conditions.


The temperature difference between the refrigerant in the evaporator and the fixture temperature affects the products stored. The TD of a fixture affects the humidity conditions inside the fixture. In order to secure the best results, with either forced air or gravity circulation, it is imperative that the proper TD be employed. TDs for food and materials which normally are refrigerated are available. The TDs required for forced air circulation are closer than those for gravity air circulation, but the same general rules apply to both.


After setting an LP switch, the operation and fixture temperature must be observed for several cycles. Particular attention must be paid to the following points:

  • The settings must not cause the condensing unit to short cycle.
  • In an installation operating on an automatic defrosting cycle, the evaporator must be completely defrosted before the switch will cut-in.
  • Avoid too great a fixture temperature difference between the cut-in and cut-out point. The fixture temperature should be maintained within ± 1.5oC of the average fixture temperature.

Remember, the range setting is equivalent to the cut-in setting and the differential set the difference between the cut-in and cutout. For this reason, the range setting, when adjusted, affects the actual setting of the cut-out point. Therefore, set the range first and then the differential.

Thanks for the question Khulani, I hope that this helps you and saves you some time.

Thanks to everybody for the overwhelming response. I receive on average over 60 questions a month and cannot publish all of them. But keep them coming, as I may answer you directly.

Looking forward to hearing from you.

Grant Laidlaw Signature


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